Wavelength-tunable lasers can be used in several applications including telecommunications and spectroscopy. The wavelength of a diode laser is usually tuned by changing the temperature and/or the electric current passing through the diode. However, the effect of the temperature and the control current on the laser wavelength is small, wherefore the aforementioned manners of tuning the wavelength of the laser are not suitable in applications that require a tuning range of several nanometers.
The wavelength of a diode laser can be changed reliably by using with the laser a separate, external cavity. The prior art comprises two widely used cavity arrangements: the Littman/Metcalf and the Littrow configurations that are disclosed in Lasers & Optronics, Continuously Tunable Diode Lasers, pp 15-17, June 1993, which is incorporated herein by reference. In both arrangements, radiation from a diode laser is collimated with a lens and directed at a grating, from which at least some of the radiation is reflected back to the laser either directly or via a mirror. In the Littrow configuration, the wavelength of a diode laser is tuned by changing the angle of the grating, for example. In the Littman/Metcalf arrangement, the wavelength of a diode laser is altered by changing the angle of the mirror.
Since adjusting the position of the mirror or the grating requires great accuracy in the Littman/Metcalf and the Littrow configurations, it is difficult to provide a desired wavelength in the prior art arrangements. They also require exact optical alignment of several components with respect to each other in the manufacturing stage of the configuration, which is difficult to achieve at least industrially and which also increases the price. Further, great demands for accuracy also decrease the durability and reliability of the configuration. Also, such prior art laser configurations are too large for many applications, since the length of the required cavity may be even several tens of centimeters.
The wavelength of a diode laser can also be tuned in a laboratory by placing in front of the laser a glass plate which operates as a dichroic mirror, and behind the laser a movable mirror. The distance of the glass plate from the laser is about 150 .mu.m and the distance of the rear mirror is about 20 .mu.m. The distance of the rear mirror can be altered piezoelectrically in this arrangement, which also results in a change in the laser wavelength. Such an arrangement is described in more detail by Zhu, X. & Cassidy, D. T. 1996, Liquid detection with InGaAsP semiconductor lasers having multiple short external cavities, Applied Optics, vol 35, no 23, pp 4689-4693, which is incorporated herein by reference. A problem with this arrangement is that the alignment of both the glass plate and the movable mirror requires a great number of components, which means that the structure cannot be integrated into one laser module in this form. Another known arrangement is a structure based on one movable mirror and integrated onto a common optical bench. This arrangement is described in more detail in Tunable laser diode using a nickel micromachined external mirror by Uenishi, Y., Honma, K. & Nagaoka, S. 1996, Electronics Letters, vol 32, no 13, pp 1207-1208, which is incorporated herein by reference. However, a problem in utilizing such an arrangement industrially is that optical benches provided with movable micromirrors are not commercially available, wherefore the arrangement cannot be utilized industrially yet. Other problems of the aforementioned arrangements, which are still at the laboratory stage, concern durability and reliability.